ω3 fatty acids, EPA and DHA, have a variety of physiological actions against circulatory diseases, neurotransmission diseases and the like, and they are used as drugs, health food, food ingredients, feed and the like. For example, a high purity EPA ethyl ester of 90 wt % or more is used as a therapeutic agent for arterial sclerosis and hyperlipidemia. In addition, beverages containing EPA or DHA are approved as food for specified health use. Glyceride and ethyl ester of EPA and DHA are utilized as dietary supplement throughout the world.
Marine fats, such as fish oil, marine algae fat and single-celled algae fat, containing EPA, DHA or other ω3 fatty acids, have a variety of types of raw material-derived impurities in addition to gum substances, phospholipids and free fatty acids, and mainly glyceride of fatty acids. EPA and DHA are mostly present therein in the form of glyceride. Most of EPA and DHA are bound to position 2 of glyceride. These marine fats are degummed or deacidified (Non-Patent Document 1) using a publicly known method.
For concentration and purification of EPA and DHA of glyceride bodies, a low-temperature solvent fractional crystallization method and a wintering method are generally used, but their efficiency for concentration is low. Thus, such techniques are often industrially used to efficiently concentrate EPA and DHA in the form of glyceride using lipase, which specifically hydrolyzes fatty acids other than the EPA or DHA bound to position 1 or 3 of the glyceride (Patent Document 1). Furthermore, in order to increase productivity, addition of reactive additives such as calcium hydroxide and magnesium chloride to reaction systems (Patent Document 2) and addition of polar solvents including lower alcohol (Patent Document 3) have been proposed.
In order to obtain higher purity EPA and DHA, a technique such as, firstly ethyl-esterifying fatty acids bound to glyceride to form a monomer is performed. As for the ethyl-esterifying step, publicly known are an enzymatic method as well as an acid catalytic method, an alkali catalytic method and the like (Patent Document 4, Non-Patent Document 2).
Highly-unsaturated fatty acids such as EPA and DHA in fat, including ethyl-esterified ω3 fatty acids, are purified at a relatively high purity using a combination method of one or more of urea addition method (Patent Documents 5 and 6), a silver nitrate complex method (Patent Documents 7 and 8), a vacuum precision distillation method including a vacuum thin-film distillation method (Patent Document 9), a chromatography method such as liquid chromatography (hereinafter, referred to as HPLC) or simulated moving bed chromatography method (Patent Document 9), and the like.
Activated charcoal, activated white earth, acid white earth, silica gel, alumina and the like are used to remove impurity components, such as foreign substances mixed during manufacture, peroxides, colored substances and odor components, from the thus-obtained high purity ω3 fatty acid ethyl esters such as EPA and DHA (Patent Documents 10 to 12).
The thus-produced high purity ω3 fatty acid ethyl esters such as EPA and DHA are produced and used as health food and drugs throughout the world, and their market currently continues to expand.
In the case of obtaining a ω3 fatty acid ethyl ester such as each of EPA and DHA as a high purity product at a high yield from a mixture of marine fat ethyl esters using prior art, for example, in the case of preparing an EPA ethyl ester of 70 wt % or more, it is necessary to remove contaminants such as all-cis-6,9,12,15-octadecatrienoic acid (18:4ω3; hereinafter, referred to as “SDA”) and DHA ethyl esters, having physicochemical characteristics similar to EPA ethyl esters. However, the removal of these matters is generally difficult, and if the reduction of these contaminants is aimed, then there will be a technical problem of reduction in the yield of the final products.
In addition, in the case of preparing a DHA ethyl ester of 70 wt % or more, the removal of SDA ethyl esters and EPA ethyl esters becomes difficult, and thus there is a technical problem of causing the reduction in the yield of final products in a similar manner.
There is a need to provide a technique of removing contaminant components including SDA in an efficient manner in order to obtain ω3 fatty acid ethyl esters such as EPA and DHA as high purity products at a high yield.